In many situations, devices must operate in potentially hazardous conditions, such as where a fuel mixture may be ignited by uncontrolled operating or environmental conditions. For example, vehicles, including aerospace vehicles, typically operate with a fuel that must be maintained in a safe condition during storage and use. The ignition hazard should be minimized even when the vehicle is subject to uncontrolled events such as an accident, electrical malfunction, a lightning strike, or static electrical discharge. Other applications requiring ignition hazard consideration include fuel transport, fuel storage, mining operations, chemical processing, metal fabrication, power plant construction and operation, and operations which involve combustible particulate such as sawdust, metal, flour, and grain.
In the aerospace industry, lightning strikes of aircraft are a concern because they could result in electrical arcs and/or heating sufficient to ignite vaporous fuel mixtures. Though lightning passes through aircraft virtually always without resulting harm, newer aircraft designs incorporating composite materials include less metal to shunt and/or dissipate the energy of a lightning strike.
Design of apparatuses exposed to ignition hazards typically involves reducing the likelihood of ignition, containing the ignition hazard, and/or withstanding the ignition hazard. Electrically conductive structures, such as fasteners, may join and/or support composite structural components within potentially combustible environments, such as within a fuel tank. These electrically conductive structures may become a focal point for electromagnetic effects (e.g., arcing, electrostatic discharge, heating, and/or hot particle ejection), e.g., due to lightning strikes.
Conventionally, metal fasteners in a composite fuel tank are isolated from the fuel volume by sealant and/or a seal cap. The sealant and/or seal cap are configured to physically and/or electrically separate the metal fastener from the fuel volume and to contain the ignition hazard. However, electromagnetic effects may generate heat and pressure transients that may damage the seal. Additionally, seals may be subject to temperature cycles due to, e.g., daily solar heating and/or operation in the atmosphere. The temperature cycling may lead to increased susceptibility to damage from electromagnetic effects and/or ignition events.